Early Triassic ichthyopterygian fossils from the Russian Far East

Ichthyopterygia is a major clade of reptiles that colonized the ocean after the end-Permian mass extinction, with the oldest fossil records found in early Spathian substage (late Olenekian, late Early Triassic) strata in the western USA. Here, we describe reptilian remains found in situ in the early Spathian Neocolumbites insignis ammonoid zone of South Primorye in the Russian Far East. Specimen NSM PV 23854 comprises fragmentary axial elements exhibiting a combination of morphological characteristics typical of Ichthyopterygia. The cylindrical centra suggest that the specimen represents a basal ichthyopterygian, and its size is comparable to that of Utatsusaurus. Specimen NSM PV 24995 is represented by a single limb bone, which is tentatively identified as an ichthyopterygian humerus. With a body length of approximately 5 m estimated from the humeral length, NSM PV 24995 represents one of the largest specimens of early Spathian marine reptiles known to date. Such size variation among the earliest ichthyopterygians might suggest an explosive diversification in size immediately after the end-Permian mass extinction. Both vertebrae and humerus specimens exhibit an extremely cancellous inner structure, suggesting a high degree of aquatic adaptation in ichthyopterygians, despite their short history of evolution in the ocean.

. Ammonoid biostratigraphy of Spathian (Olenekian, Lower Triassic) ichthyopterygian and possible ichthyopterygian fossil localities. Ammonoid biostratigraphic data were compiled after Ji et al. 7 for Chaohu (South China); Tongtherm et al. 5 for Phatthalung (Thailand); Zakharov et al. 20,22 and Shigeta and Kumagae 73 for South Primorye (Russia); Bando and Shimoyama 59 , Ehiro et al. 61 , Shigeta and Nakajima 29 , and Shigeta 60 for Kitakami (Northeast Japan); Hurum et al. 11 and Hansen et al. 12 for Svalbard (Norway); Guex et al. 56 and Brayard et al. 55 for the Western United States; Tozer 57 for British Columbia (Canada). Ichthyopterygian horizons are also compiled after Ji et al. 7 for Chaohu; Tongtherm et al. 4 for Phatthalung; Shikama et al. 32 for Kitakami; Maxwell and Kear 10 , Hurum et al. 11 , Hansen et al. 17 , Engelschiøn et al. 18 , and Ekeheien et al. 16 for Svalbard; Massare and Callaway 14 , Scheyer et al. 2 , Kelley et al. 15 , Smith et al. 3 for Western USA; Brinkman et al. 38 and Cuthbertson et al. 12,39 for British Columbia. Silhouettes indicate the occurrence of ichthyopterygian fossils. Possible source of materials without reliable ammonoid biostratigraphic information is indicated with a question mark. Taxonomic names are given in generic or higher rank with approximate body length when available. Asterisks indicate disputed taxa. The Grippia Niveau is likely from either Bajarunia euomphala or Parasibirites grambergi zone. www.nature.com/scientificreports/ ichthyopterygian Omphalosaurus 10,16 . Grippia niveau is dated as the lower Spathian (Bajarunia euomphala zone) or middle Spathian (Parasibirites grambergi zone) 17 (Fig. 1). From Lower Saurian niveau (upper Spathian, Keiserlingites subrobustus zone) of the same member in the Marmierfjellet of Spitzbergen, many other fragmentary reptilian fossils including large ichthyopterygians (Pessopteryx, Cymbospondylus and disputed Isfjordosaurus) and Omphalosaurus 10,18 are found. As mentioned above, recent studies have gradually elucidated the diversity of early Mesozoic marine reptiles. However, the paleobiogeographic information of marine reptiles through the Early Triassic is still scant. Moreover, the early stage of evolution toward the pelagic lifestyle in Ichthyopterygia is poorly understood. Here, we report the occurrence of two ichthyopterygian specimens: NSM (National Museum of Nature and Science, Tokyo) PV 23854 and NSM PV 24995, from the Lower Triassic strata of Cape Zhitkov, Russky Island, in the Russian Far East. With detailed biostratigraphic data, these specimens were collected in situ from the lower Spathian Neocolumbites insignis zone, which is regarded as the same age as the Procolumbites beds in southern China, from which the oldest known specimen of Chaohusaurus was reported 6,7,19 . The large size and three-dimensional preservation of the new skeletal elements would help to reveal the process of gigantism and aquatic adaptation in Ichthyopterygia based on functional histology. Geological setting. The materials were found in calcareous concretions collected in situ from the lower part of the Spathian Zhitkov Formation of Cape Zhitkov, Russky Island, South Primorye (43° 1′ 21.8′′ N, 131° 56′ 3.8″ E; Fig. 2). The Zhitkov Formation conformably overlies the lower Spathian Schmidt Formation (Tirolites-Amphistephanites ammonoid zone) and shows a fining-upward sedimentary sequence (Fig. 3). The beds of the Zhitkov Formation are numbered from 55 to 68, and the sequence is divided into the Neocolumbites insignis (beds [55][56][57][58][59][60][61][62][63] and Subfengashites (formerly Subcolumbites) multiformis (beds 64-67) ammonite zones, which are overlain by the Anisian Karazin Formation 20 . According to new data obtained from Tchernyschev and Paris bays 21,22 , the uppermost Olenekian Prohungarites sp.-Paranorellina parisi beds have been proposed for the uppermost part of the Zhitkov Formation. Therefore, the Prohungarites sp.-Paranorellina parisi beds are not discovered in the Cape Zhitkov section.

Materials
The sedimentary facies of the lower Zhitkov Formation represent sandy mudstone with interbedded sandstone, indicating a continental slope depositional environment 23 . A long-snouted temnospondyl amphibian (Aphaneramma sp.) has been reported in bed 59 20,24 . There are also occurrences of other Triassic ichthyopterygian fossils in the Russian Far East, including Lower Triassic specimens briefly noted in the literature; however, many of these materials were likely lost without being described [25][26][27] . In a nearby locality, a durophagous ichthyopterygian, Tholodus sp. has been reported in the Acrochordiceras kiparisovae zone (Middle Anisian) of the overlying Karazin Formation 22,27 .
Specimens NSM PV 23854 and 24995 are from beds 58 and 60, respectively, both of which belong to the N. insignis zone of the lower Zhitkov Formation. This zone best correlates with the Procolumbites beds of southern China (Fig. 1), which yielded an early ichthyopterygian, Chaohusaurus 7 . Ovtcharova et al. 28 and Shigeta and Nakajima 29 considered the Procolumbites zone in South China to be "lower" Spathian, whereas Ji et al. 7 regarded  Referred specimen. NSM PV 23854 comprises two nearly complete vertebral centra (' A' and 'B'), which are both likely from the dorsal position of the vertebral column (see Remarks for anatomical identification), neural arch (we include the neural spine while using this term), and rib head. These elements were extracted from a single concretion, and their three-dimensional structure is well preserved, although some parts are missing due to www.nature.com/scientificreports/ weathering and other post-burial processes. Therefore, the rib shaft and the neural spine rim were reconstructed using glue to merge the fragments in position.
Description. Viewed laterally, the centra of NSM PV 23854 are approximately square in shape; the height/ length is 30.8/25.5 mm in centrum ' A' (Fig. 4f) and 32.0/29.5 mm in centrum 'B' (Fig. 4j). The anterior and posterior intervertebral joint surfaces of each centrum are almost evenly hexagonal, moderately concave near the anterior and posterior margins, and strongly concave near the center, with the middle portions perforated by a small notochordal canal (Fig. 4g,h,k). In both specimens, the centrum contributes to the ventral half of the diapophysis, and the parapophysis is lost or confluent with the diapophysis. These characteristics suggest that the single-headed rib is attached across the neurocentral suture line and near the intervertebral disk. The anterior margin of the diapophysis is truncated, and the transverse processes laterally project from the anterodorsal corners (Fig. 4i). The precise outline of centrum 'B' cannot be observed in the ventral view because it is deformed by a microfault; however, it is approximately square and somewhat constricted at the center owing to anterolateral and posterolateral projections (Fig. 4m). Two longitudinal subparallel ridges are present on the ventral surface of the centra (Fig. 4i,m). The floor of the neural canal is an hourglass-shaped depression (Fig. 4l). The sutural facets of the neural arch are smooth, showing no signs of neurocentral suture fusion (Fig. 4l). The neural arch possesses posterolaterally-projecting transverse processes (Fig. 4c). The ventral border of the transverse process reaches the neurocentral suture (Fig. 4a). Viewed laterally, the base of the neural arch is convex (Fig. 4a) and matches the neurapophysis of centra ' A' and 'B'; therefore, the neural arch may have belonged to www.nature.com/scientificreports/ either of the vertebrae. The neural arch narrows at the level of the spinal cord in the lateral view and is anteroposteriorly broad in the neural spine (Fig. 4a). Pre-and post-zygapophyses are poorly preserved because of weathering, but it is likely that the original left and right zygapophyses were not distinct, as in the derived Triassic ichthyosaur Shastasaurus pacificus (originally S. altispinus) 30 (Fig. 4b,d). The marginal region of the neural spine also appears to be missing and reconstructed, making it difficult to determine its precise outline. However, it is certain that the neural spine is laterally thin, slightly curved anteriorly, and at least 56.5 mm high when measured vertically from the level of the neurocentral suture line ( Fig. 4a-d). According to the reconstructed outline of the neural spine, it was likely not much higher than the above measurement, and thus no more than twice as high as the centrum. The size and shape of the dorsal vertebra is close to that of the dorsal region of U. hataii 31 .
The rib fragment expands proximally, with a single-headed proximal end and a longitudinally grooved shaft, similar to the dorsal ribs of Utatsusaurus 31 (Fig. 4e). Although the middle portion of the rib fragment is missing because of weathering, we confirm that the rib shaft is slender and curved (Fig. 4e).
Remarks. Deeply biconcave centra are commonly observed in Triassic and Jurassic ichthyopterygians, occasionally with a tiny central perforation 30,31 . Previous research has suggested that the depth of the centrum concavity is shallow in U. hataii from the late Spathian 32 ; however, further preparation and observation of the holotype (IGPS 95941a) and more recent specimens (NSM PV 20028, 21865, UHR 30691) revealed that the vertebrae are deeply amphicoelous in this species 29 (also confirmed by personal observation by Y.N.). The neural arch and centrum are rarely fused in ichthyopterygians 33 and separate in the current material also. In fact, none of these characteristics have been solely defined as a synapomorphy of Ichthyopterygia (or Ichthyosauria). For example, a notochordal canal is a plesiomorphy commonly found in basal tetrapods 34 and amniotes 35,36 . However, the combination of the above-cited characteristics (lack of neurocentral suture fusion, deep amphicoelous centrum with notochordal canal, indistinct zygapophyses, and extremely cancellous bone) has not been reported for any tetrapods other than Ichthyopterygia. In non-ichthyopterygian ichthyosauriforms (i.e., Nasorostra), the centrum anatomy has not been reported in detail. As the vertebral centrum is not exposed or well preserved in published specimens of Cartorhynchus lenticarpus and Sclerocormus parviceps (personal observation by Y.N.), Nasorostra is postulated as a candidate; however, the associated rib is gracile as in general ichthyopterygian ribs, and not robust or pachyostotic as in nasorostrans 1,13 . Therefore, we identified NSM PV 23854 as Ichthyopterygia gen. et sp. indet.
Cylindrical (anteroposteriorly long) dorsal centra are found in Californosaurus, mixosaurids, and basal ichthyopterygians such as Chaohusaurus, Grippia, and Utatsusaurus, which are all from the Triassic 37 . In contrast, post-Triassic derived lineages and a few large Triassic groups, including Cymbospondylus and Shonisaurus, generally possess shortened dorsal centra. The ratio of the height of the anterior dorsal neural arch relative to the centrum height is less than 2.0 in NSM PV 23854, and less than that of Gulosaurus specimens found in British Columbia (between 2.2 and 2.9) 38,39 and mixosaurids (between 2.0 and 3.0 or higher) 40 . In addition, the microanatomical characteristics (see "Bone microanatomy and histology" section) of the current specimen are typical of those observed in basal, long-tailed ichthyopterygians 41 . Therefore, NSM PV 23854 likely belongs to the basal lineage, at least according to the above morphology. However, this conclusion is not absolute because simplified zygapophyses, which are observed in the derived lineages of Triassic ichthyopterygians 42 , might be present in NSM PV 23854.
A single articular facet crossing the neurocentral suture is common among the posterior cervical and dorsal vertebrae of Triassic ichthyopterygians. This condition is present in the dorsal vertebra of Triassic forms such as Utatsusaurus (personal observation by Y.N.), Cymbospondylus, Shastasaurus, Mixosaurus, Besanosaurus, and Californosaurus 30,32,43 . Furthermore, the dorsal vertebral centra of Cymbospondylus sp. from the Spathian of Spitzbergen 18 (originally identified as an ichthyopterygian taxon Merriamosaurus hulkei 44,45 , which is currently disputed) shows anteriorly truncated diapophysis 18 as in NSM PV 23854, although the height/length ratio of Spitzbergen Cymbospondylus centra is almost 2.0, unlike in NSM PV 23854, which has a height/length ratio of almost 1.0. In addition, the associated rib can be identified as a dorsal rib based on its curved shape 46,47 . Therefore, it is likely that the centra and neural arch of NSM PV 23854 are from the dorsal region.

Ichthyopterygia gen. et sp. indet.
Referred specimen. The material NSM PV 24995 is a single limb bone that is morphologically similar to the humerus of the Middle Triassic ichthyosaurian Cymbospondylus youngorum LACM DI 157871 48 . The specimen was originally broken into two pieces in the shaft; however, the broken surfaces were fit together to reconstruct the precise total length. Locality and horizon. Bed 60 of the Zhitkov Formation (Neocolumbites insignis zone, lower Spathian, Lower Triassic) 20 , exposed along the northern shoreline of Cape Zhitkov, Russky Island, South Primorye, Russia.
Description. NSM PV 24995 is a limb bone that is approximately dumbbell-shaped and slightly curved ( Fig. 5a-d). The specimen is 131 mm long proximodistally; the anteroposterior widths of the smaller and larger ends are 56 and 62 mm, respectively, and the anteroposterior width of the diaphyseal shaft is 39 mm. Both epiphyses exhibit boundaries between the endochondral and periosteal regions, but not many pronounced trochanters, tuberosities, or articular facets. The smaller epiphysis forms a wedge-shape with a projection at a side (Fig. 5f), whereas the larger epiphysis appears uneven triangle (Fig. 5e), both in the end views. A triangular epiphysis has been identified in the humeral proximal end of a basal ichthyosaurian Cymbospondylus youngorum from the Anisian, in Nevada, and each of its triangle corners corresponds to anterior margin, posterior margin, www.nature.com/scientificreports/ and deltopectoral crest, respectively 49 (Fig. 6). Therefore, this specimen was tentatively identified as a humerus with a triangular proximal end with a shallow deltopectoral process on the ventral side. A small projection in the ventral side of distal metaphysis likely corresponds to the "postero-distal tuberosity" in Motani 50 , which determines the posterior (ulnar) side of the humerus. Thus, we consider NSM PV 24995 as a right humerus. Both anterior and posterior margins of the diaphysis are concave as in humerus of Cymbospondylus petrinus and C. youngorum 48,49 . The humerus of Cymbospondylus duelferi also shows constricted diaphysis; however, it has a much broader distal end than proximal end 49 . The anterior margin of NSM PV 24995 is smoothly curved, forming a very shallow and thick anterior flange (Fig. 5a-c). The anterior flange appears to direct anterodorsally in the proximal view, anteriorly in the midshaft in the anterior view, and somewhat dorsally in the distal view (Fig. 5b,e,f). Such directional change in the flange results in a dome-shaped curvature of the anterior part of the humerus. Similar curvature is known from the convex humeral margin of Pessopteryx nisseri from the Spathian (e.g., PMO 229.780, Engelschiøn et al. 18 , Fig. 4C3 3 ), although Engelschiøn et al. 18 determined this convex margin of the humerus as posterior and the concave as anterior, and one of the proximal projections as "dorsal process". However, the anatomical orientation of P. nisseri humerus lacks consensus partly because this genus is known only from isolated skeletal elements; in fact, Motani 50 comprehensively compared ichthyopterygian humeri and determined the convex margin of Pessopteryx humerus as anterior, and a proximal projection as a deltopectoral crest. Here we follow Motani 50 and point similarity in the curvature of the anterior flange between NSM PV 24995 and P. nisseri, despite the shortened and rounded shape of the latter. The anterior margin of NSM PV 24995 is thickened so that the flange possesses two distinct ridges, while the posterior margin is a sharp, single ridge. The humeral head is indistinct, but a shallow postero-proximal corner might correspond to it (Fig. 5a,c,d). www.nature.com/scientificreports/ The deltopectoral crest is close to the shaft axis as in C. petrinus and C. youngorum, but not anteriorly displaced as in Grippia longirostris and Mixosaurus natans 50 (Fig. 5a,b,d,e). The ventral aspect of NSM PV 24995 shows a shallow depression near the postero-proximal corner and a small and sharp indentation slightly proximal to the postero-distal tuberosity, but their function is unknown (Fig. 5a,d). In the dorsal surface, a weakly developed, rugose crest distributes in the proximal metaphysis, nearly parallel to the proximal diaphyseal margin, and continues to a shallow tubercle in the posterior margin (Fig. 5c). Also, a very shallow groove develops near the parallel to the antero-posterior region in the dorsal aspect (Fig. 5c).
Remarks. Motani et al. 50 categorized ichthyopterygian humeri into morphotypes 1-3, with the exceptional Cymbospondylus buchseri (very short humerus with smoothly concave and anteriorly directed anterior flange and complete anterior and posterior shafts), C. petrinus (the elongate version of C. buchseri), and Merriamia zitteri types. Among these types, NSM. PV 24995 shares some characters with type 1 (complete posterior shaft as a basal condition) and 2 (pronounced postero-distal tuberosity as one of the derived conditions). The anterior flange is fundamentally short, but it is present both proximally and distally, not only distally as in Type 3 and M. zitteri. Therefore, NSM PV 24995 is most comparable to C. petrinus type, although the postero-distal tuberosity is more pronounced than in C. petrinus. The function of the rugose crest is unknown, but it could be interpreted as the insertion for dorsal muscles. It is also unknown what the postero-distal shallow groove is for, but it would be a rudimentary entepicondylar groove, which is not common among ichchyopterygians 35 . The concave anterior flange is a symplesiomorphic character in Ichthyosauromorpha found in basal members such as hupehsuchians, Cartorhynchus and Chaohusaurus 1,50-52 ; however, similar condition is also found in more derived taxa including Cymbospondylus, Pessopteryx, and Shonisaurus 1,48,49 . In considering that NSM PV 24995 shares other characters with Cymbospondylus and shastasaurids, it is most reasonable when we assume that the current specimen belongs to Ichthyopterygia. www.nature.com/scientificreports/ Bone microanatomy and histology. The centra exhibit internal cancellous bone in the endochondral and periosteal domains, the trabeculae of which are widely spaced in the latter. In the mid-sagittal section, the endochondral domain forms an X-shape that expands from the growth center toward the edges of the anterior and posterior intervertebral surfaces (Fig. 7a). The endochondral domain also expands from the growth center toward the right and left neurocentral contact surfaces, as observed in the mid-transverse section (Fig. 7b). The intervertebral surfaces and the neurocentral contact surfaces are covered with a very thin layer of calcified cartilage, which represents the growth front of the endochondral domain, comprised secondary trabecular spongy bone (Fig. 7c,e,g). The periosteal domain comprises a loose network of trabecular bone surrounded by thin  and (d,f,g,h) lights. Enlarged areas (c-j) are indicated by rectangles in (a-d) www.nature.com/scientificreports/ (0.2-0.5 mm) periosteal bone (Fig. 7h,i). The trabeculae in the periosteal domain have longitudinal and radial orientations in the sagittal and coronal sections, respectively, evoking a three-dimensional reticular network. The thin periosteal bone comprises longitudinally oriented parallel-fibered bone tissue and is associated with longitudinally oriented and concentrically aligned vascular canals (Fig. 7h-j). The centra are anteroposteriorly perforated in the middle, forming notochordal canals (Fig. 7c,d). Notochordal canals are surrounded by a bony tube lined by a thin layer of calcified cartilage (Fig. 7c-f). The fracture surface of the mid-diaphysis of the NSM PV 24995 humerus shows an extremely cancellous inner organization with thin trabeculae and a small proportion of peripheral compact cortex (Fig. 8). The inner two-thirds of the cross-section are cemented with carbonate minerals. The outer zone consists of cancellous bone with thin trabeculae and represents the only peripheral 1-2-mm-thick layer of compact bone, but the bone compactness in the internal zone is hardly recognizable from the fracture surface due to the cementation (Fig. 8b). The compact bone is highly vascularized with longitudinal and radial canals, where the former exhibit a circumferential arrangement. The vascular canals are transformed into larger intertrabecular spaces in the internal region. In the computed tomography (CT) scan images, we confirm that the whole internal region is filled with cancellous bone (Fig. 8c-f), and the endochondral domain appears to expand from the growth center toward both ends, making an hourglass shape; the surrounding rest corresponds to the periosteal region (Fig. 8e,f). Nutrient canals run from the growth center to the bone surface, both in periosteal and endochondral domains (Fig. 8e,f).

Discussion
Stratigraphic range of early ichthyopterygians. In each of the seven regions with Early Triassic marine reptile fossils, the lowermost occurrence of ichthyopterygian is no older than the early Spathian (Tirolites, Procolumbites, Neocolumbites, Columbites, and the upper part of the Bajarunia euomphala beds or (sub) zones), when this unit is present (Fig. 1). In the case of Western USA, the oldest record of Ichthyopterygia is approximately 249 Ma, in the early early Spathian 28,53 , approximately 3 million years after the P-T boundary. Therefore, the diversification and geological radiation of ichthyopterygians occurred quite rapidly because it was not until the P-T boundary that they appeared in a marine environment 54 . Hitherto, large ichthyopterygians that appear to be longer than 5 m have been reported in and above the middle Spathian or are poorly dated 2,16 .
Outside USA, records of middle-sized marine reptiles are possibly from the lower Spathian in Japan (see below). The ammonoid Columbites and the biostratigraphically younger Subcolumbites are mutually exclusive www.nature.com/scientificreports/ in the western United States and southern China 55,56 , although these zones are not found in British Columbia 57 (Fig. 1). Some previous studies have noted that Utatsusaurus ranges only within the Subcolumbites zone and not in the Columbites zone 2,5 , partly because of the misleading definition of Columbites and Subcolumbites subzones within the Subcolumbites zone sensu Bando and Shimoyama 58 , which is clearly not equivalent to the currently accepted middle Spathian Subcolumbites zone. To avoid confusion, we refer to the Subcolumbites zone sensu Bando and Shimoyama (1974) as the "Columbites-Subcolumbites zone" in this study. Several ichthyopterygian materials, including the U. hataii holotype (IGPS 95941) and paratype (IGPS 95942), come from the upper part of the "Columbites-Subcolumbites zone" (lower-middle Spathian) and "Arnautoceltites zone" (upper Spathian) of the Osawa Formation 32,59 . However, some "Utatsusaurus" materials have been found in the lower part of the "Columbites-Subcolumbites zone" in the Osawa Formation of Utatsu-Tatezaki (specimen "nos. A-C") 32 . Because these fossils are not described or illustrated, their actual affinity is unknown, although they might be comparable to Utatsusaurus materials in terms of their size and shape, given that Shikama et al. 32 assigned them to this genus.
A recent study re-identified several ammonoid fossils from the type section of the Osawa Formation as Hellenites tchernyschewiensis, H. inopinatus, Neocolumbites grammi, N. insignis, Procolumbites ussuriensis, and P. subquadratus in its lower part; therefore, the lower Osawa Formation can be correlated to the N. insignis zone of South Primorye 60 . It is possible that Utatsu-Tatezaki, the other ichthyosauromorph-bearing "Columbites-Subcolumbites zone" locality, which is the type locality of Utatsusaurus, also includes N. insignis (upper lower Spathian) and Subcolumbites (middle Spathian) subzones because no large missing parts have been recognized between the underlying Hiraiso Formation and Osawa Formation 32,61,62 . As evidenced by Smith et al. 3 and this study, some medium (~ 2.5 m) to large (~ 5 m) ichthyopterygians were present during the early Spathian, shortly after the P-T boundary (approximately 3 Mys 53 ), thus, it is not surprising that lower Spathian marine reptiles exist among Japanese collections.
According to the diversity of Hupehsuchia, Nasorostra, and basal species of Ichthyopterygia in the Spathian of southern China, Motani et al. 1 suggested that Ichthyosauromorpha originated in this area. The diverse ichthyosauromorphs in the Nanlinghu Formation are assumed to represent one of the oldest records of Ichthyosauromorpha (Fig. 1). To date, hupehsuchians and nasorostrans have been reported exclusively in South China; however, rib specimens reported from the Osawa Formation (UMUT MV 31051) show a thickened shaft and a broad head that closely resembles that of a large nasorostran S. parviceps 13,46 . Therefore, further investigation of incomplete materials from the lower Spathian in Japan and the Russian Far East are required to better understand the origin of ichthyosauromorph groups.
Body size and ecology of Early Triassic marine reptiles. Marine reptile body size increased rapidly during the Early Triassic 2,13 . As noted above, the vertebral size of the oldest known ichthyopterygian from Tirolites beds reaches ~ 3.5 cm in diameter 3 , which presumably corresponds to the largest vertebra of a ~ 3-m-long ichthyopterygian. Other lower Spathian ichthyopterygians are generally small; Chaohusaurus is approximately 1 m long and smaller than the largest known contemporary predatory fish 2,5 . Thaisaurus chonglakmanii is known from a single specimen TF 2454, consisting of an almost complete skull and incomplete postcranial skeleton, and is close to C. geishanensis in size and shape 4,37 . The cranial length of T. chonglakmanii is ~ 10 cm, and the estimated body length is 53-84 cm based on the skull/body proportion in embryonic and adult C. geishanensis 4,5 .
Although the growth stage of the T. chonglakmanii holotype has not been estimated precisely, the ossification stage of carpal bones in the only specimen is close to that of adult C. geishanensis 4,63 . Thus, T. chonglakmanii is likely a small ichthyopterygian less than 1 m long.
The new vertebrae from the Russian Far East are similar in size to the mid-dorsal vertebrae of U. hataii UHR 30691, which reached a total body length (TL) of 2.6 m 31 , although the precise TL of NSM PV 23854 is hard to predict. The body size of NSM PV 24995 is not known either; however, the humrus length (HL) is approximately twice that of U. hataii, whose humeri are 61 mm and 66 mm long based on UHR 30691, neglecting post-burial deformation (see Motani et al. 31 regarding the necessity of retro-deformation). Furthermore, linear regression analysis on the relationship between HL and TL (both in meter) by Sander et al. 48 found a relationship of log 10 (TL) = 1.599 (s.e. = 0.038) + 1.023 (s.e. = 0.035) × log 10 (HL), with R 2 = 0.9801 48 . Accordingly, we speculate that a suitable TL of NSM PV 24995 (HL = 0.131 = ~ 10 -0.88 ) is approximately 5.0 (~ 10 0.7 ) m. An upper Spathian 'ichthyosaur humerus' NMMNH P-65886 is even approximately twice as long as NSM PV 24995 2 . As noted above, the identity of this specimen is controversial 2,13 , but the original identification is convincing because of a significant dorsoventral thickness, distinct proximal and distal ends, and unambiguous deltopectoral crest and notched anterior flange 2 , as in NSM PV 24995. If we regard NMMNH P-65886 as a humerus, ichthyopterygians likely reached a gigantic body length of ~ 11 m until the middle-late Spathian 2 . The increasing body size trend of Ichthyopterygia is well recognized in the Grippia Niveau (lower Spathian) to Lower Saurian Niveau (upper Spathian) of Spitzbergen 16,18 . The size of ichthyopterygians possibly reached ~ 3 m in the age of lower Spathian Tirolites zone as evidenced by a vertebral centrum measuring ~ 3.5 cm in diameter 3 , which is slightly larger than the largest Utatsusaurus centra (~ 3 cm in NSM PV 20028, TL = ~ 2.6 m). In Hupehsuchia and Sauropterygia, a few fossils from the Early Triassic exhibit exceptionally large body sizes, although these materials are not well dated [64][65][66] . Overall, the trend toward gigantism in Ichthyopterygia became obvious before the end of the Spathian around the world, and the discovery of early Spathian large (total body length > ~ 5 m) marine reptiles from the eastern margin of Panthalassa (this study) suggests that such gigantism started, perhaps globally, before the middle Spathian.
The body sizes of osteichthyan fish, on the other hand, drastically increased across the Middle-Late Permian boundary, but exhibited no significant change across the P-T boundary 67  www.nature.com/scientificreports/ 1.85-m-long Birgeria americana from Nevada 2,68 . Subsequently, the body size of osteichthyan fish decreased across the Early-Middle Triassic boundary, shortly after large marine reptiles appeared in the fossil record 67 . This size reduction in Osteichthyes could be considered as the result of their competition with marine reptiles. The gigantism of ichthyopterygians during the Spathian might be due to their physiological advances over fish, including rapid growth and a high metabolic rate, as evidenced by high vascularization in parallel-fibered bone tissue or fibrolamellar complex 69,70 . Furthermore, oceanic environments underwent fluctuating anoxic to oxygenated conditions during the Early Triassic, as suggested by many authors 71,72 ; however, air-breathing marine reptiles were not likely as severely disturbed by oceanic anoxia-dysoxia as fish, at least in terms of respiration (although it should be noted that some osteichthyans are capable of lung breathing as well as gill breathing). Among reptiles and other amniotes, the loose spongious organization of bones is frequently observed in highly advanced swimmers and divers, such as modern whales. In contrast, increased bone mass is common among semi-aquatic forms like sea otters and poorly active shallow divers like sirenians 73 . The limb bones filled with loose spongious tissue have been reported from the marine turtles, especially the pelagic and deep-diving species Dermochelys coriacea [74][75][76] . Ichthyopterygian bone compactness has a considerable variation 41,70 , and the current specimens are particularly cancellous. In addition, radial nutrient canals in the endochondral domain are also observed in marine turtles Dermochelys and Archelon 76 , as well as in Pessopteryx 77 and derived plesiosaurs 78 . Therefore, the bone microanatomy in NSM PV 23854 and NSM PV 24995 suggests that these reptiles were already highly adapted to an aquatic lifestyle, supporting the hypothesis that the ichthyosauromorph ancestor had already adapted very rapidly to the oceanic environment by the early Spathian 54 . The sedimentary facies of marine reptile-bearing beds in the Lower Zhitkov Formation (sandy mudstone with intercalated turbidite sandstone) represent a continental slope depositional environment 23 , and the paleogeographic position of South Primorye faced the western Panthalassa ocean in the Early Triassic 55 . These support the observation that the ichthyopterygians reported in this study were well adapted to life in the open ocean; however, it is also possible that the carcasses were transported from shallow marine by turbidity currents.

Methods
Thin sections were prepared from centrum specimens (NSM PV 23854) using a standard procedure described previously 79 . Transverse and mid-sagittal sections were taken from centra 'a' and 'b' respectively, so that the sectional planes passed through the growth center. Thin sections were observed under normal and cross-polarized transmitted light using a Leica DMLP polarizing microscope. Thin-section photographs were taken macroscopically using an EPSON GT-X980 image scanner and microscopically using a Leica DFC420 digital camera attached to the microscope. A long bone (NSM PV 24995) was originally broken into two pieces in the shaft. The shaft was reconstructed using epoxy adhesive after the sectional plane was photographed for microanatomical observation using an image scanner (EPSON GT-X980). Reconstructed specimen NSM PV 24995 was CT-scanned using inspeXio SMX-225CT FPDHR micro-focus X-ray system (Shimadzu Co.) in National Museum of Nature and Science (Tsukuba, Japan), 225 kV, 70 μA, 167.45 μm/voxel.

Data availability
All data generated or analyzed during this study are included in this published article.